Functions

Functions are self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to “call” the function to perform its task when needed.

Swift’s unified function syntax is flexible enough to express anything from a simple C-style function with no parameter names to a complex Objective-C-style method with local and external parameter names for each parameter. Parameters can provide default values to simplify function calls and can be passed as in-out parameters, which modify a passed variable once the function has completed its execution.

Every function in Swift has a type, consisting of the function’s parameter types and return type. You can use this type like any other type in Swift, which makes it easy to pass functions as parameters to other functions, and to return functions from functions. Functions can also be written within other functions to encapsulate useful functionality within a nested function scope.

Defining and Calling Functions

When you define a function, you can optionally define one or more named, typed values that the function takes as input (known as parameters), and/or a type of value that the function will pass back as output when it is done (known as its return type).

Every function has a function name, which describes the task that the function performs. To use a function, you “call” that function with its name and pass it input values (known as arguments) that match the types of the function’s parameters. A function’s arguments must always be provided in the same order as the function’s parameter list.

The function in the example below is called sayHello, because that’s what it does—it takes a person’s name as input and returns a greeting for that person. To accomplish this, you define one input parameter—a String value called personName—and a return type of String, which will contain a greeting for that person:

funcsayHello(personName: String) -> String {

letgreeting = "Hello, " + personName + "!"

returngreeting

}

All of this information is rolled up into the function’s definition, which is prefixed with the func keyword. You indicate the function’s return type with the return arrow-> (a hyphen followed by a right angle bracket), which is followed by the name of the type to return.

The definition describes what the function does, what it expects to receive, and what it returns when it is done. The definition makes it easy for the function to be called unambiguously from elsewhere in your code:

println(sayHello("Anna"))

// prints "Hello, Anna!"

println(sayHello("Brian"))

// prints "Hello, Brian!"

You call the sayHello function by passing it a String argument value in parentheses, such as sayHello("Anna"). Because the function returns a String value, sayHello can be wrapped in a call to the println function to print that string and see its return value, as shown above.

The body of the sayHello function starts by defining a new String constant called greeting and setting it to a simple greeting message for personName. This greeting is then passed back out of the function using the return keyword. As soon as return greeting is called, the function finishes its execution and returns the current value of greeting.

You can call the sayHello function multiple times with different input values. The example above shows what happens if it is called with an input value of "Anna", and an input value of "Brian". The function returns a tailored greeting in each case.

To simplify the body of this function, combine the message creation and the return statement into one line:

funcsayHelloAgain(personName: String) -> String {

return"Hello again, " + personName + "!"

}

println(sayHelloAgain("Anna"))

// prints "Hello again, Anna!"

Function Parameters and Return Values

Function parameters and return values are extremely flexible in Swift. You can define anything from a simple utility function with a single unnamed parameter to a complex function with expressive parameter names and different parameter options.

Multiple Input Parameters

Functions can have multiple input parameters, which are written within the function’s parentheses, separated by commas.

This function takes a start and an end index for a half-open range, and works out how many elements the range contains:

funchalfOpenRangeLength(start: Int, end: Int) -> Int {

returnend - start

}

println(halfOpenRangeLength(1, 10))

// prints "9"

Functions Without Parameters

Functions are not required to define input parameters. Here’s a function with no input parameters, which always returns the same String message whenever it is called:

funcsayHelloWorld() -> String {

return"hello, world"

}

println(sayHelloWorld())

// prints "hello, world"

The function definition still needs parentheses after the function’s name, even though it does not take any parameters. The function name is also followed by an empty pair of parentheses when the function is called.

Functions Without Return Values

Functions are not required to define a return type. Here’s a version of the sayHello function, called sayGoodbye, which prints its own String value rather than returning it:

funcsayGoodbye(personName: String) {

println("Goodbye, \(personName)!")

}

sayGoodbye("Dave")

// prints "Goodbye, Dave!"

Because it does not need to return a value, the function’s definition does not include the return arrow (->) or a return type.

Note

Strictly speaking, the sayGoodbye function does still return a value, even though no return value is defined. Functions without a defined return type return a special value of type Void. This is simply an empty tuple, in effect a tuple with zero elements, which can be written as ().

The return value of a function can be ignored when it is called:

funcprintAndCount(stringToPrint: String) -> Int {

println(stringToPrint)

returncount(stringToPrint)

}

funcprintWithoutCounting(stringToPrint: String) {

printAndCount(stringToPrint)

}

printAndCount("hello, world")

// prints "hello, world" and returns a value of 12

printWithoutCounting("hello, world")

// prints "hello, world" but does not return a value

The first function, printAndCount, prints a string, and then returns its character count as an Int. The second function, printWithoutCounting, calls the first function, but ignores its return value. When the second function is called, the message is still printed by the first function, but the returned value is not used.

Note

Return values can be ignored, but a function that says it will return a value must always do so. A function with a defined return type cannot allow control to fall out of the bottom of the function without returning a value, and attempting to do so will result in a compile-time error.

Functions with Multiple Return Values

You can use a tuple type as the return type for a function to return multiple values as part of one compound return value.

The example below defines a function called minMax, which finds the smallest and largest numbers in an array of Int values:

funcminMax(array: [Int]) -> (min: Int, max: Int) {

varcurrentMin = array[0]

varcurrentMax = array[0]

forvalueinarray[1..<array.count] {

ifvalue < currentMin {

currentMin = value

} elseifvalue > currentMax {

currentMax = value

}

}

return (currentMin, currentMax)

}

The minMax function returns a tuple containing two Int values. These values are labeled min and max so that they can be accessed by name when querying the function’s return value.

The body of the minMax function starts by setting two working variables called currentMin and currentMax to the value of the first integer in the array. The function then iterates over the remaining values in the array and checks each value to see if it is smaller or larger than the values of currentMin and currentMax respectively. Finally, the overall minimum and maximum values are returned as a tuple of two Int values.

Because the tuple’s member values are named as part of the function’s return type, they can be accessed with dot syntax to retrieve the minimum and maximum found values:

letbounds = minMax([8, -6, 2, 109, 3, 71])

println("min is \(bounds.min) and max is \(bounds.max)")

// prints "min is -6 and max is 109"

Note that the tuple’s members do not need to be named at the point that the tuple is returned from the function, because their names are already specified as part of the function’s return type.

Optional Tuple Return Types

If the tuple type to be returned from a function has the potential to have “no value” for the entire tuple, you can use an optional tuple return type to reflect the fact that the entire tuple can be nil. You write an optional tuple return type by placing a question mark after the tuple type’s closing parenthesis, such as (Int, Int)? or (String, Int, Bool)?.

Note

An optional tuple type such as (Int, Int)? is different from a tuple that contains optional types such as (Int?, Int?). With an optional tuple type, the entire tuple is optional, not just each individual value within the tuple.

The minMax function above returns a tuple containing two Int values. However, the function does not perform any safety checks on the array it is passed. If the array argument contains an empty array, the minMax function, as defined above, will trigger a runtime error when attempting to access array[0].

To handle this “empty array” scenario safely, write the minMax function with an optional tuple return type and return a value of nil when the array is empty:

funcminMax(array: [Int]) -> (min: Int, max: Int)? {

ifarray.isEmpty { returnnil }

varcurrentMin = array[0]

varcurrentMax = array[0]

forvalueinarray[1..<array.count] {

ifvalue < currentMin {

currentMin = value

} elseifvalue > currentMax {

currentMax = value

}

}

return (currentMin, currentMax)

}

You can use optional binding to check whether this version of the minMax function returns an actual tuple value or nil:

ifletbounds = minMax([8, -6, 2, 109, 3, 71]) {

println("min is \(bounds.min) and max is \(bounds.max)")

}

// prints "min is -6 and max is 109"

Function Parameter Names

All of the above functions define parameter names for their parameters:

funcsomeFunction(parameterName: Int) {

// function body goes here, and can use parameterName

// to refer to the argument value for that parameter

}

However, these parameter names are only used within the body of the function itself, and cannot be used when calling the function. These kinds of parameter names are known as local parameter names, because they are only available for use within the function’s body.

External Parameter Names

Sometimes it’s useful to name each parameter when you call a function, to indicate the purpose of each argument you pass to the function.

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

funcsomeFunction(externalParameterNamelocalParameterName: Int) {

// function body goes here, and can use localParameterName

// to refer to the argument value for that parameter

}

Note

If you provide an external parameter name for a parameter, that external name must always be used when you call the function.

As an example, consider the following function, which joins two strings by inserting a third “joiner” string between them:

funcjoin(s1: String, s2: String, joiner: String) -> String {

returns1 + joiner + s2

}

When you call this function, the purpose of the three strings that you pass to the function is unclear:

join("hello", "world", ", ")

// returns "hello, world"

To make the purpose of these String values clearer, define external parameter names for each join function parameter:

In this version of the join function, the first parameter has an external name of string and a local name of s1; the second parameter has an external name of toString and a local name of s2; and the third parameter has an external name of withJoiner and a local name of joiner.

You can now use these external parameter names to call the function unambiguously:

join(string: "hello", toString: "world", withJoiner: ", ")

// returns "hello, world"

The use of external parameter names enables this second version of the join function to be called in an expressive, sentence-like manner by users of the function, while still providing a function body that is readable and clear in intent.

Note

Consider using external parameter names whenever the purpose of a function’s arguments would be unclear to someone reading your code for the first time. You do not need to specify external parameter names if the purpose of each parameter is unambiguous when the function is called.

Shorthand External Parameter Names

If you want to provide an external parameter name for a function parameter, and the local parameter name is already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead, write the name once, and prefix the name with a hash symbol (#). This tells Swift to use that name as both the local parameter name and the external parameter name.

This example defines a function called containsCharacter, which defines external parameter names for both of its parameters by placing a hash symbol before their local parameter names:

Default Parameter Values

You can define a default value for any parameter as part of a function’s definition. If a default value is defined, you can omit that parameter when calling the function.

Note

Place parameters with default values at the end of a function’s parameter list. This ensures that all calls to the function use the same order for their non-default arguments, and makes it clear that the same function is being called in each case.

Here’s a version of the join function from earlier, which provides a default value for its joiner parameter:

funcjoin(strings1: String, toStrings2: String,

withJoinerjoiner: String = " ") -> String {

returns1 + joiner + s2

}

If a string value for joiner is provided when the join function is called, that string value is used to join the two strings together, as before:

join(string: "hello", toString: "world", withJoiner: "-")

// returns "hello-world"

However, if no value of joiner is provided when the function is called, the default value of a single space (" ") is used instead:

join(string: "hello", toString: "world")

// returns "hello world"

External Names for Parameters with Default Values

In most cases, it is useful to provide (and therefore require) an external name for any parameter with a default value. This ensures that the argument for that parameter is clear in purpose if a value is provided when the function is called.

To make this process easier, Swift provides an automatic external name for any parameter that has a default value. The automatic external name is the same as the local name, as if you had written a hash symbol before the local name in your code.

Here’s a version of the join function from earlier, which does not provide external names for any of its parameters, but still provides a default value for its joiner parameter:

funcjoin(s1: String, s2: String, joiner: String = " ") -> String {

returns1 + joiner + s2

}

In this case, Swift automatically provides an external parameter name for the joiner parameter. The external name must therefore be provided when calling the function, making the parameter’s purpose clear and unambiguous:

join("hello", "world", joiner: "-")

// returns "hello-world"

Note

You can opt out of this behavior by writing an underscore (_) instead of an explicit external name when you define the parameter. However, external names for parameters with default values are preferred.

Variadic Parameters

A variadic parameter accepts zero or more values of a specified type. You use a variadic parameter to specify that the parameter can be passed a varying number of input values when the function is called. Write variadic parameters by inserting three period characters (...) after the parameter’s type name.

The values passed to a variadic parameter are made available within the function’s body as an array of the appropriate type. For example, a variadic parameter with a name of numbers and a type of Double... is made available within the function’s body as a constant array called numbers of type [Double].

The example below calculates the arithmetic mean (also known as the average) for a list of numbers of any length:

funcarithmeticMean(numbers: Double...) -> Double {

vartotal: Double = 0

fornumberinnumbers {

total += number

}

returntotal / Double(numbers.count)

}

arithmeticMean(1, 2, 3, 4, 5)

// returns 3.0, which is the arithmetic mean of these five numbers

arithmeticMean(3, 8.25, 18.75)

// returns 10.0, which is the arithmetic mean of these three numbers

Note

A function may have at most one variadic parameter, and it must always appear last in the parameter list, to avoid ambiguity when calling the function with multiple parameters.

If your function has one or more parameters with a default value, and also has a variadic parameter, place the variadic parameter after all the defaulted parameters at the very end of the list.

Constant and Variable Parameters

Function parameters are constants by default. Trying to change the value of a function parameter from within the body of that function results in a compile-time error. This means that you can’t change the value of a parameter by mistake.

However, sometimes it is useful for a function to have a variable copy of a parameter’s value to work with. You can avoid defining a new variable yourself within the function by specifying one or more parameters as variable parameters instead. Variable parameters are available as variables rather than as constants, and give a new modifiable copy of the parameter’s value for your function to work with.

Define variable parameters by prefixing the parameter name with the keyword var:

This example defines a new function called alignRight, which aligns an input string to the right edge of a longer output string. Any space on the left is filled with a specified padding character. In this example, the string "hello" is converted to the string "-----hello".

The alignRight function defines the input parameter string to be a variable parameter. This means that string is now available as a local variable, initialized with the passed-in string value, and can be manipulated within the body of the function.

The function starts by working out how many characters need to be added to the left of string in order to right-align it within the overall string. This value is stored in a local constant called amountToPad. If no padding is needed (that is, if amountToPad is less than 1), the function simply returns the input value of string without any padding.

Otherwise, the function creates a new temporary String constant called padString, initialized with the pad character, and adds amountToPad copies of padString to the left of the existing string. (A String value cannot be added on to a Character value, and so the padString constant is used to ensure that both sides of the + operator are String values.)

Note

The changes you make to a variable parameter do not persist beyond the end of each call to the function, and are not visible outside the function’s body. The variable parameter only exists for the lifetime of that function call.

In-Out Parameters

Variable parameters, as described above, can only be changed within the function itself. If you want a function to modify a parameter’s value, and you want those changes to persist after the function call has ended, define that parameter as an in-out parameter instead.

You write an in-out parameter by placing the inout keyword at the start of its parameter definition. An in-out parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value.

You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified. You place an ampersand (&) directly before a variable’s name when you pass it as an argument to an inout parameter, to indicate that it can be modified by the function.

Note

In-out parameters cannot have default values, and variadic parameters cannot be marked as inout. If you mark a parameter as inout, it cannot also be marked as var or let.

Here’s an example of a function called swapTwoInts, which has two in-out integer parameters called a and b:

funcswapTwoInts(inouta: Int, inoutb: Int) {

lettemporaryA = a

a = b

b = temporaryA

}

The swapTwoInts function simply swaps the value of b into a, and the value of a into b. The function performs this swap by storing the value of a in a temporary constant called temporaryA, assigning the value of b to a, and then assigning temporaryA to b.

You can call the swapTwoInts function with two variables of type Int to swap their values. Note that the names of someInt and anotherInt are prefixed with an ampersand when they are passed to the swapTwoInts function:

varsomeInt = 3

varanotherInt = 107

swapTwoInts(&someInt, &anotherInt)

println("someInt is now \(someInt), and anotherInt is now \(anotherInt)")

// prints "someInt is now 107, and anotherInt is now 3"

The example above shows that the original values of someInt and anotherInt are modified by the swapTwoInts function, even though they were originally defined outside of the function.

Note

In-out parameters are not the same as returning a value from a function. The swapTwoInts example above does not define a return type or return a value, but it still modifies the values of someInt and anotherInt. In-out parameters are an alternative way for a function to have an effect outside of the scope of its function body.

Function Types

Every function has a specific function type, made up of the parameter types and the return type of the function.

For example:

funcaddTwoInts(a: Int, b: Int) -> Int {

returna + b

}

funcmultiplyTwoInts(a: Int, b: Int) -> Int {

returna * b

}

This example defines two simple mathematical functions called addTwoInts and multiplyTwoInts. These functions each take two Int values, and return an Int value, which is the result of performing an appropriate mathematical operation.

The type of both of these functions is (Int, Int) -> Int. This can be read as:

“A function type that has two parameters, both of type Int, and that returns a value of type Int.”

Here’s another example, for a function with no parameters or return value:

funcprintHelloWorld() {

println("hello, world")

}

The type of this function is () -> (), or “a function that has no parameters, and returns Void.” Functions that don’t specify a return value always return Void, which is equivalent to an empty tuple in Swift, shown as ().

Using Function Types

You use function types just like any other types in Swift. For example, you can define a constant or variable to be of a function type and assign an appropriate function to that variable:

varmathFunction: (Int, Int) -> Int = addTwoInts

This can be read as:

“Define a variable called mathFunction, which has a type of ‘a function that takes two Int values, and returns an Int value.’ Set this new variable to refer to the function called addTwoInts.”

The addTwoInts function has the same type as the mathFunction variable, and so this assignment is allowed by Swift’s type-checker.

You can now call the assigned function with the name mathFunction:

println("Result: \(mathFunction(2, 3))")

// prints "Result: 5"

A different function with the same matching type can be assigned to the same variable, in the same way as for non-function types:

mathFunction = multiplyTwoInts

println("Result: \(mathFunction(2, 3))")

// prints "Result: 6"

As with any other type, you can leave it to Swift to infer the function type when you assign a function to a constant or variable:

letanotherMathFunction = addTwoInts

// anotherMathFunction is inferred to be of type (Int, Int) -> Int

Function Types as Parameter Types

You can use a function type such as (Int, Int) -> Int as a parameter type for another function. This enables you to leave some aspects of a function’s implementation for the function’s caller to provide when the function is called.

Here’s an example to print the results of the math functions from above:

This example defines a function called printMathResult, which has three parameters. The first parameter is called mathFunction, and is of type (Int, Int) -> Int. You can pass any function of that type as the argument for this first parameter. The second and third parameters are called a and b, and are both of type Int. These are used as the two input values for the provided math function.

When printMathResult is called, it is passed the addTwoInts function, and the integer values 3 and 5. It calls the provided function with the values 3 and 5, and prints the result of 8.

The role of printMathResult is to print the result of a call to a math function of an appropriate type. It doesn’t matter what that function’s implementation actually does—it matters only that the function is of the correct type. This enables printMathResult to hand off some of its functionality to the caller of the function in a type-safe way.

Function Types as Return Types

You can use a function type as the return type of another function. You do this by writing a complete function type immediately after the return arrow (->) of the returning function.

The next example defines two simple functions called stepForward and stepBackward. The stepForward function returns a value one more than its input value, and the stepBackward function returns a value one less than its input value. Both functions have a type of (Int) -> Int:

funcstepForward(input: Int) -> Int {

returninput + 1

}

funcstepBackward(input: Int) -> Int {

returninput - 1

}

Here’s a function called chooseStepFunction, whose return type is “a function of type (Int) -> Int”. chooseStepFunction returns the stepForward function or the stepBackward function based on a Boolean parameter called backwards:

funcchooseStepFunction(backwards: Bool) -> (Int) -> Int {

returnbackwards ? stepBackward : stepForward

}

You can now use chooseStepFunction to obtain a function that will step in one direction or the other:

varcurrentValue = 3

letmoveNearerToZero = chooseStepFunction(currentValue > 0)

// moveNearerToZero now refers to the stepBackward() function

The preceding example determines whether a positive or negative step is needed to move a variable called currentValue progressively closer to zero. currentValue has an initial value of 3, which means that currentValue > 0 returns true, causing chooseStepFunction to return the stepBackward function. A reference to the returned function is stored in a constant called moveNearerToZero.

Now that moveNearerToZero refers to the correct function, it can be used to count to zero:

println("Counting to zero:")

// Counting to zero:

whilecurrentValue != 0 {

println("\(currentValue)... ")

currentValue = moveNearerToZero(currentValue)

}

println("zero!")

// 3...

// 2...

// 1...

// zero!

Nested Functions

All of the functions you have encountered so far in this chapter have been examples of global functions, which are defined at a global scope. You can also define functions inside the bodies of other functions, known as nested functions.

Nested functions are hidden from the outside world by default, but can still be called and used by their enclosing function. An enclosing function can also return one of its nested functions to allow the nested function to be used in another scope.

You can rewrite the chooseStepFunction example above to use and return nested functions: